This invention relates generally to flexible drive devices and more particularly to flexible drive, recanalizing catheters for intravascular surgery.
In U.S. Pat. No. 4,445,509 (Auth) there is disclosed a catheter apparatus for recanalizing (opening) a passageway, e.g., an artery, which has been occluded by intraarterial deposits of atherosclerotic plaque. That recanalization catheter includes a multi-fluted, rotary cutting head mounted at the distal end of the catheter and arranged to be rotated by a flexible drive shaft extending down the center of the catheter. The drive shaft is arranged to be rotated within the catheter by an electric motor coupled to the proximal end thereof. The drive shaft is disclosed as being a steel helical coil of approximately 0.05 inch (1.3 mm) diameter. Such a coil is stated in the patent to be successful in transmitting high rotational speed (greater than 25,000 rpm) in a controlled fashion and with mechanical security.
Other prior art devices utilizing flexible drive shafts for conveying rotary power to a working head or tool are disclosed in the following U.S. Pat. Nos.: 1,481,078 (Albertson), 1,636,038 (Bolozsky et al.), 2,570,335 (Fitch), 2,761,297 (Buchsteiner et al.) and 4,424,045 (Kulischenko et al.).
In order for a recanalizing catheter to have wide applicability of use in intravascular surgery, its length should be sufficiently large, e.g., 2 to 3 feet or more, while its outside diameter, at least adjacent the working end, is sufficiently small, e.g., 3-4 mm. Moreover the catheter should be able to bend through a minimum diameter radius of curvature of 3 inches or less, in order to reach small, remotely located restrictions, e.g., occlusions.
As will be appreciated by those skilled in the art the torsional shear stress produced on a flexible drive shaft (e.g., a wire) will differ for different composition wires, e.g., approximately 150,000 psi for steel wires, 70,000 psi for beryllium-copper wires. If the radius of curvature through which the drive shaft must bend is very small, e.g., less than 3 inches, high bending stresses will be induced therein. In order to reduce bending strain the diameter of the flexible drive shaft or wire must be made very small, e.g., 0.02 or less inches. If the restriction opening tool is to be operated at a high rate of speed, e.g., greater than 20,000 rpm, in order to provide sufficient power at low torque, the deleterious dynamic effects of critical whirl and friction caused by high side loads on the bearing surfaces supporting the drive wire must be overcome or minimized while the positional neutrality (centering) of the drive shaft is maintained in order to insure that proper operation ensues. The flexible drive systems of the prior art as set forth above appear to leave much to be desired from the standpoint of effectiveness and efficiency of operation in applications involving high speed, small diameter, and small radius of curvature.